[cairo] Proposed simplified replacement for Cairo compositing operators.

Carl Worth cworth at cworth.org
Fri Oct 22 11:55:33 PDT 2004

On Wed, 20 Oct 2004 22:02:15 -0700, Bill Spitzak wrote:
> I have also minimal support for non-premultiplied surfaces.  Although 
> unnecessary for photographic work, Non-premultiplied images are needed for 
> GUI.

I'm not sure I'm convinced of that.

>      This is primarily so that 8-bit images can be made that fade as expected
> into a background of a given color, and compositing the same image atop 
> itself many times can approach any color, rather than some subset defined by 
> the level of alpha.

It sounds to me like these sorts of operation would naturally be
described as an opaque source composited with an alpha mask. If so, then
using premultiplied images would not pose a problem.

But, we would need a way to set a mask operand in cairo in order to be
able to easily express this kind of operation, (ie. without temporary
surfaces). I had already been considering adding something like that, so
it's something we should keep in mind as we work on specifying the
imaging model.


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